The enzyme 5-lipoxygenase (5-LOX) initiates the synthesis of pro-inflammatory leukotrienes. These lipid mediators are synthesized from arachidonic acid (AA) released from the bilayer by the action of Ca2+-dependent phospholipase A2. 5-LOX activity is short-lived, and temporal control appears in part due to an intrinsic instability of the enzyme. This instability provides a mechanism for auto-regulation, preventing an over-production of pro-inflammatory leukotrienes. However, programmed obsolescence is not common to all lipoxygenases, and stable isoforms have been identified. We propose to address three critical aspects of control of 5-LOX activity: (1) Product specificity: The substrate for 5-LOX is the polyunsaturated eicosanoid arachidonic acid. The first step of the reaction is the abstraction of hydrogen from the central carbon of a pentadiene. AA has three pentadiene moieties (and six possible sites of peroxidation, each with either R- or S- chirality). Yet animal lipoxygenases generally produce a single, regio- and stereo- specific product. We will develop a model for 5-LOX specificity that is consistent with its product specificty. We have a 2.86E resolution structure of an engineered 5-LOX that establishes the foundation for these biochemical and structural studies. (2) Programmed obsolescence Programmed obsolescence in 5-LOX appears to have two components: structural instability and turnover-based suicide inhibition. Our data, including our stable mutant form of 5-LOX, suggest that features unique to 5-LOX result in a tenuously restrained C- terminus that contributes to 5-LOX instability. Experiments to define the molecular basis for non- turnover and turnover-based inactivation are proposed. (3) Compartmentalization Ca2+- dependent membrane binding of 5-LOX targets the enzyme to substrate reservoirs and promotes proximity to downstream enzyme activities. Experimental data support a model in which specific Ca2+ binding sites stabilize insertion loops in the C2-like domain of 5-LOX. Others have suggested that 5-LOX binds to its helper protein FLAP, an integral membrane protein. We propose experiments to define the interaction of 5-LOX with the bilayer and determine whether the catalytic domain interacts with the membrane as well, and whether FLAP hands off the substrate to the enzyme, or simply concentrates the AA in the membrane.